A location where an x-ray beam hits a sample is called an x-ray spot. In some applications, especially if the x-ray source is used with polycapillary focusing optics, it can be important for the x-ray spot to be stable (i.e. does not fluctuate over time). Temperature changes of, and temperature differentials within, the x-ray source can cause instability or fluctuation of the x-ray spot.
It would be beneficial to increase positional stability of the x-ray spot. Large x-ray tubes can have large pathways for removal of heat, but some x-ray tubes (e.g. for a portable x-ray source) need small size, and can't afford large pathways for removal of heat. Therefore, it can be more difficult to provide x-ray spot stability in these small x-ray tubes. X-ray tubes are often cooled by a heat exchanger carrying a liquid coolant. Such a cooling method may be impractical for a portable x-ray source.